The term matrix potential of the soil describes the force per unit area with which the water contained therein is bound. The matrix potential is therefore normally given in the unit Pascal with a negative sign. However, also the term "extraction potential" ("Saugapannung"), which is given with a positive sign, is frequently used.
DE 39 11 151 A1 discloses a device that can be used for measuring the matrix potential in the soil. This device consists essentially of a tubular and gastight measuring cell which is to be inserted in the soil. The measuring cell is initially filled with water which, when said measuring cell has been inserted in the soil, leaks into the soil through a porous, liquid-permeable section of the measuring cell in accordance with the matrix potential prevailing in the soil. Due to the fact that part of the water leaks out of the measuring cell, a measurable negative pressure is generated in said measuring cell and this negative pressure is used as a measure of the matrix potential prevailing in the soil.
German-Offenlegungsschrift 16 73 046 discloses a device by means of which the soil moisture can be measured. The device comprises a measuring cell which is to be inserted in the soil and which has arranged therein electrodes separated by a dielectric having a known porosity and pore size. The capacitance prevailing between the electrodes, whose magnitude is determined by the nature of the dielectric used, is evaluated by an adequate measurement circuit. In view of the fact that soil moisture penetrates into the dielectric, the effective permittivity ratio or dielectric constant, which determines the capacitance, is changed. The determinable change of capacitance is regarded as a measure of the moisture in the soil. The dielectric provided in the measuring cell preferably consists of ceramics.
It is known that, especially in the field of agriculture, it is more important to measure the matrix potential of a soil than to measure of the moisture thereof A fine-grained soil consisting of a specific material has, due to a larger volume-absorbing surface per unit volume, a better capability of binding the water contained therein than a soil consisting of the same material but having a larger grain size. Provided that both types of soil contain the same amount of moisture, plants in the soil having the larger grain size will be able to absorb the water contained in said soil more easily.
The device known from DE 39 11 151 A1 is therefore more suitable for agricultural applications than the device known from German-Offenlegungsschrift 16 73 046. The device known from DE 39 11 151 A1 is, however, based on a principle permitting a determination of the matrix potential essentially only in a measurement change between 0 to approx. -800 Pascal, this being a range that exists only in the case of very wet soils.